In many applications, electric discharge machining is used for the mass production of parts which must be machined to close tolerances. There are several factors which affect the repeatability of accuracy of machining by the EDM process including the process parameters of electrode size, spark energy, spark current, gap length and gap voltage. For example, when the EDM process is used to machine a small hole through a thin plate, the variation in diameter of the wire electrode over a large number of parts may result in many parts being out of tolerance unless special measures are taken. Also, mechanical wear in the fixturing system will result in variation in hole diameters. Even though such variations may be on the order of one ten thousandth inch, they nevertheless adversely affect the accuracy of the finished part. In the case of an orifice plate for fuel injectors, for example, the specified tolerance on the hole size is one percent but the electrode wire commercially available has a tolerance of two percent. Thus, the desired accuracy cannot be achieved uniformly by conventional practice.
Accordingly, there is a need for obtaining improved uniformity and accuracy by the EDM process. Further, it should be achieved without depending upon stringent control of the various parameters to precisely constant values. Instead, as a practical matter, the improved accuracy must be achieved while tolerating variations with time in certain operating parameters of the EDM process.
A general object of this invention is to provide an improved method and apparatus for electric discharge machining to obtain a high degree of accuracy and repeatable accuracy in the machining of parts.